1. Field of the Invention
The present invention relates to an organic electroluminescent device, which can emit light by converting electric energy to light (hereinafter, also referred to as “organic EL device”, “luminescent device” or “EL device”).
2. Description of the Related Art
Organic EL devices have been attracting attention as promising display devices because highly luminescent emission is obtained from these devices at a low voltage. However, these organic EL devices generally have lower luminous efficiency than, for instance, inorganic LED devices. Accordingly, organic EL devices more improved in luminous efficiency and luminance have been needed.
The external energy efficiency indicating the luminous efficiency of an organic EL device is given by the product of the internal energy efficiency and the light-extraction efficiency of the device (see, for example, “Optics Letters” (1997), vol. 22, No. 6, page 396, the disclosure of which is incorporated by reference herein.). In order to improve the luminous efficiency of an organic EL device, it is necessary to improve the light-extraction efficiency as well as the internal energy efficiency.
The light-extraction efficiency means the ratio of luminescence emitted to the air from the front side of a transparent substrate of a device to the luminescence of the device. Before the luminescence in a luminescent layer is emitted to the air, it must pass through the boundaries of several media differing in refractive index. According to Snell's refraction law, light incident to each boundary at an angle larger than the critical angle of the boundary is wholly reflected on the boundary, propagates in a layer and then disappears or is emitted from the side surface of the layer, so that the amount of light emitted from the front of the device is reduced by that amount. As a consequence, the front luminance is reduced when the device is applied to, for example, a display.
As a method of improving the reduction in front luminance, a method is known in which a diffraction grating made of dots and grooves is formed at the boundary to allow light to diffract, thereby extracting light (see, for example, Japanese Patent No. 2991183, the disclosure of which is incorporated by reference herein.). In this case, the degree of improvement in the front luminance is insufficient and also, a rainbow color appears due to the interference of reflected light, and this method is therefore undesirable.
An attempt to eliminate this rainbow color by random dots has been recently reported (see, for example, “Preprints, (The 51st Meeting 2004), Japan Society of Applied Physics & Related Society, 30a-ZN-13, the disclosure of which is incorporated by reference herein.). This trial succeeded in eliminating light interference. However, the degree of improvement in the front luminance is decreased and remains unsatisfactory.
As a method of improving the reduction in front luminance, a method in which a prism is disposed at the boundary is known (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-86353, the disclosure of which is incorporated by reference herein.). This method greatly improves the front luminance because the prism has a light converging function.
Although there is the problem that an image formed through a prism is blurred, leading to reduced contrast, this can be improved by decreasing the distance between the luminescent layer and the prism and by decreasing the distance (pitch) between the apexes of the prisms.
However, if these prisms are formed regularly and the pitch of these prisms is decreased, the same problem as in the case of the aforementioned diffraction grating, specifically, the aforementioned rainbow color problem caused by the interference of reflected light arises. In this situation, no method that can solve this rainbow color problem has been found that does not also decrease the degree of improvement in luminance.